A fuel cell is a generation device that converts chemical energy of fuel into electrical energy through electrochemical reaction within a stack, rather than changing the chemical energy into heat through combustion. Fuel cells may be used to supply power for small electric/electronic products, such as portable devices, as well as power for industrial purposes, household purposes, and for driving vehicles.
Currently, among fuel cells, a polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) having the highest power density has been actively researched as a power source for driving vehicles. It has been found that a PEMFC has a fast start time and fast power conversion reaction time due to a low operation temperature. The PEMFC includes a membrane electrode assembly (MEA) including a solid polymer electrolyte membrane in which hydrogen ions migrate and catalytic electrode layers, in which an electrochemical reaction occurs, attached on both sides of the solid polymer electrolyte membrane, a gas diffusion layer (GDL) serving to evenly distribute reaction gases and transmit generated electrical energy, a gasket and a fastening mechanism maintaining airtightness of reaction gases and a coolant and appropriate fastening pressure, and a bipolar plate moving the reaction gases and the coolant.
When a fuel cell stack is assembled using such unit cell components, the combination of the MEA and the GDL, key components, is positioned in the innermost position within the cell. The MEA has catalytic electrode layers formed on both sides of the polymer electrolyte membrane and having a catalyst coated thereon such that hydrogen and oxygen are reacted, namely, an anode and a cathode, and the GDL, the gasket, and the like, are stacked on outer portion of the anode and the cathode. The bipolar plate, which supplies a reaction gas (e.g., hydrogen as fuel and oxygen as an oxidizing agent or air) and has a flow field through which a coolant passes, is positioned on an outer side of the GDL. This configuration forms a unit cell, and after a plurality of unit cells are stacked, a current collector, an insulating plate, and end plates supporting stacked cells are coupled to the outermost side. The unit cells are repeatedly stacked between the end plates and fastened to form a fuel cell stack.
In order to obtain a potential required in an actual vehicle, unit cells need to be stacked to correspond to required potential (a stack is a plurality of stacked unit cells). Potential generated in a single unit cell is about 1.3V, and in order to produce power required for driving vehicle, a plurality of cells are stacked in series. However, a fuel cell stack typically degrades over time. Thus, an expected output corresponding to an acceleration pedal amount may not be generated in a fuel cell electric vehicle (FCEV) due to a diminished fuel cell stack, thereby degrading running performance.